1. Field of the Invention
The present invention relates to a process for producing chlorine. More particularly, the present invention relates to a process for producing chlorine by oxidizing hydrogen chloride with oxygen, wherein said process can produce chlorine by using a catalyst having high activity in a smaller amount at a lower reaction temperature. The above invention also relates to a process for producing chlorine by oxidizing hydrogen chloride, wherein said process can facilitate control of the reaction temperature by making it easy to remove the reaction heat from catalyst bed using a catalyst having good thermal conductibility, which can be formed by containing a compound having high thermal conductivity of a solid phase, and can achieve high reaction conversion by keeping the whole catalyst bed at sufficient temperature for industrially desirable reaction rate.
The present invention also relates to a process for producing a supported ruthenium oxide catalyst. More particularly, the present invention relates to a process for producing a supported ruthenium oxide catalyst, wherein said process is a process for producing a catalyst having high activity and can produce a catalyst having high activity capable of producing the desired compound by using a smaller amount of the catalyst at a lower reaction temperature.
Furthermore, the present invention relates to a supported ruthenium oxide catalyst. The present invention relates to a supported ruthenium oxide catalyst, wherein said catalyst has high activity and can produce the desired compound by using a smaller amount of the catalyst at a lower reaction temperature.
2. Description of the Related Art
It is well known that chlorine is useful as a raw material of vinyl chloride, phosgene, etc., and can be produced by oxidizing hydrogen chloride. For example, the Deacon reaction by using a Cu catalyst is well known. For example, British Patent No. 1,046,313 discloses a process for oxidizing hydrogen chloride by using a catalyst containing a ruthenium compound, and also discloses that ruthenium (III) chloride is particularly effective among the ruthenium compounds. Furthermore, a process for supporting a ruthenium compound on a carrier is also disclosed and, as the carrier, silica gel, alumina, pumice and ceramic material are exemplified. As the Example, a ruthenium chloride catalyst supported on silica is exemplified. However, a test was conducted using a catalyst prepared by using a process for preparing a ruthenium (III) chloride supported on silica disclosed in said patent publication. As a result, the ruthenium compound as a catalyst component is drastically volatilized and it was disadvantageous for industrial use. For example, European Patent EP-0184413A2 discloses a process for oxidizing hydrogen chloride by using a chromium oxide catalyst. However, conventionally known processes had a problem that the activity of the catalyst is insufficient and high reaction temperature is required.
When the activity of the catalyst is low, a higher reaction temperature is required but the reaction of oxidizing hydrogen chloride with oxygen to produce chlorine is an equilibrium reaction. When the reaction temperature is high, it becomes disadvantageous in view of equilibrium and the equilibrium conversion of hydrogen chloride decreases. Therefore, when the catalyst has high activity, the reaction temperature can be decreased and, therefore, the reaction becomes advantageous in view of equilibrium and higher conversion of hydrogen chloride can be obtained. In case of the high reaction temperature, the activity is lowered by volatilization of the catalyst component. Also in this point of view, it has been required to develop a catalyst which can be used at low temperature.
Both high activity per unit weight of catalyst and high activity per unit weight of ruthenium contained in the catalyst are required to the catalyst, industrially. Since high activity per unit weight of ruthenium contained in the catalyst can reduces the amount of ruthenium contained in the catalyst, it becomes advantageous in view of cost. It is possible to select the reaction condition which is more advantageous in view of equilibrium by conducting the reaction at a lower temperature using a catalyst having high activity. It is preferred to conduct the reaction at a lower temperature in view of stability of the catalyst.
The catalyst used in the oxidizing reaction of hydrogen chloride includes, for example, a supported ruthenium oxide catalyst prepared by supporting ruthenium chloride on a carrier, drying the supported one, heating in a hydrogen gas flow to form a supported metal ruthenium catalyst, and oxidizing the catalyst. When ruthenium chloride is reduced with hydrogen, sintering of ruthenium occurs, which results in decrease of activity of the resulting catalyst.
A process for preparing ruthenium oxide supported on a carrier without causing sintering of ruthenium during the preparation step of a catalyst is preferred. First, a process has been desired which is not a process for reducing at high temperature by using hydrogen, but a process for preparing ruthenium oxide on a carrier with preventing sintering by treating a ruthenium compound with a mixture of a basic compound and a reducing compound, or a mixture of an alkali compound and a reducing compound, and oxidizing the treated one.
Second, a process has been desired which is a process for preparing ruthenium oxide on a carrier with preventing sintering by oxidizing after passing through a state of an oxidation number of 1 to less than 4 valence without preparing a ruthenium compound having an oxidation number of 0 valence by completely reduction
Third, it has been desired to develop a catalyst preparing process which can obtain a highly active hydrogen chloride oxidizing catalyst by passing through a preparation of a highly dispersed supported metal ruthenium catalyst, when the preparation is carried out by supporting a ruthenium compound on a carrier, reducing the supported one in order to prepare supported metal ruthenium catalyst, and oxidizing to prepare a supported ruthenium oxide catalyst.
A supported ruthenium oxide catalyst obtained by using an anatase crystalline or non-crystalline titanium oxide as a carrier was highly active to oxidation of hydrogen chloride, but it has been required to develop a catalyst having higher activity.
In the case of a conventional carrier which the content of an OH group on the surface of titanium oxide is too large or small, a catalyst having high activity was not obtained and the catalytic activity decreased sometimes as time passed.
When the oxidizing reaction of hydrogen chloride is conducted at a higher reaction rate with conventionally known catalysts, heat generated as a result of the high reaction rate can not be sufficiently removed and the temperature of the catalyst bed increases locally and, therefore, the reaction temperature can not be easily controlled.
Furthermore, when the reaction is conducted by using these catalysts, a large temperature distribution occurs in the catalyst bed and it is impossible to keep the whole system at sufficient temperature for industrially desirable reaction rate without exceeding upper temperature limit for keeping high catalyst activity. Therefore, the reaction conversion is lowered.
As a process for increasing the rate of removing heat generated during the reaction, for example, a process for increasing a heat transfer area in contact with external coolant per volume of the catalyst bed is known. However, when the heat transfer area becomes large, the cost of a reactor increases. On the other hand, when heat is removed by cooling the catalyst bed from outside, heat transfers to an external coolant through the catalyst bed and the heat transfer surface. When the thermal conductivity of the catalyst is improved, the heat removing rate increases. Therefore, it has been required to develop a catalyst having good thermal conductibility, which can increase the heat removing rate, to avoid difficulty of control of the reaction temperature.
It is generally considered that, when a carrier supporting an active component of the catalyst is mixed with an inactive component at the ratio of 1:1, the activity per volume or per weight reduced to half. Therefore, it is required to develop a catalyst having good thermal conductivity as described above and further to develop a catalyst having high activity which the activity of the catalyst per volume or per weight does not decrease.
It is known that, since a supported catalyst is generally prepared by supporting on a carrier having porediameters of from 30 to 200 angstroms, the rate-determining step of the reaction is controlled by the catalyst pore diffusion control and it is difficult to improve the activity of the catalyst. Therefore, it has been required to develop a catalyst having macropores which the inside of the catalytic particles can be utilized.
As a result, since the reaction proceeds in the vicinity of the outer surface of the catalytic particles, it is considered that ruthenium oxide supported on the outer surface of the carrier is used in the reaction but ruthenium oxide supported in the catalytic particles is not used in the reaction. Therefore, it has been required to develop a catalyst obtained by supporting ruthenium oxide on the outer surface of the catalyst.
It is also known that a ruthenium oxide catalyst is useful as a catalyst in process for preparing chlorine by an oxidizing reaction of hydrogen chloride and is obtained by hydrolyzing ruthenium chloride, oxidizing the hydrolyzed one, and calcining the oxidized one. For example, European patent EP-0743277A1 discloses that a ruthenium oxide catalyst supported on titanium oxide is obtained by hydrolyzing a ruthenium compound by using an alkali metal hydroxide, supporting the hydrolyzed one on titanium hydroxide, and calcining the supported one under air. The present inventors have found that the supported ruthenium oxide catalyst is obtained by oxidizing a supported metal ruthenium catalyst. As a process for preparing the supported metal ruthenium catalyst, for example, it is known that a process for preparing a supported metal ruthenium catalyst by supporting ruthenium chloride on a carrier, drying the supported one, and heating the dried one in a hydrogen gas flow. However, there was a problem that a supported ruthenium oxide catalyst prepared by oxidizing a catalyst reduced by hydrogen has low activity due to sintering of ruthenium when ruthenium chloride is reduced with hydrogen.
A process for preparing ruthenium oxide supported on a carrier with preventing sintering has been required. First, a process has been desired which is not a process for reducing at high temperature by using hydrogen, but for treating a ruthenium compound with a mixture of a reducing compound and a basic compound, or a mixture of an alkali compound and a reducing compound, and oxidizing the treated one.
Second, a process has been desired which is a process for preparing ruthenium oxide on a carrier with preventing sintering by oxidizing after passing through a state of an oxidation number of 1 to less than 4 valence without preparing a ruthenium compound having an oxidation number of 0 valence by completely reduction.
In general, it is difficult to reduce the ruthenium compound with a reducing compound, unlike platinum and palladium. For example, because of this, there is a problem that a supported ruthenium oxide catalyst prepared by oxidizing after adding hydrazine to ruthenium chloride has low activity because of a formation of complex by adding hydrazine to ruthenium chloride.
A supported ruthenium oxide catalyst obtained by using an anatase crystalline or non-crystalline titanium oxide as a carrier was highly active to oxidation of hydrogen chloride, but it has been required to develop a catalyst having higher activity.
In the case of a content of an OH group on the surface of titanium oxide which is a conventional carrier is too large or small, a catalyst having high activity was not obtained and the catalytic activity decreased sometimes as time passed.
It is known that the rate-determining step of the reaction is under the catalyst pore diffusion control and it is difficult to improve the activity of the catalyst since a supported catalyst is generally prepared by supporting on a carrier having pore diameters of from 30 to 200 angstroms. As a result, it is considered that ruthenium oxide supported on the outer surface of the carrier is used in the reaction but ruthenium oxide supported in the catalytic particles is not used in the reaction since the reaction proceeds in the vicinity of the outer surface of the catalytic particles. Therefore, it has been required to develop a technique for supporting ruthenium oxide on the outer surface of the catalyst.